In recent years, with growing awareness of global environmental issues, there has been an increasing interest in a thermoelectric cooling element using the Peltier effect as a CFC-free cooling device. Also, there has been an increasing interest a thermoelectric generation device providing a power generation system using unused waste heat energy and using the Seebeck effect to reduce the amount of emission of carbon dioxide.
A performance index Z of a thermoelectric conversion material is represented by the following formula (1): Z α2/(ρκ) . . . (1), where α is the Seebeck coefficient of the thermoelectric conversion material, ρ is the electric resistivity of the thermoelectric conversion material, and κ is the thermal conductivity of the thermoelectric conversion material. Z has the dimension of an inverse of the temperature and becomes a dimensionless value when multiplied by the absolute temperature T. The ZT value is called a dimensionless performance index and thermoelectric conversion efficiency increases with a thermoelectric conversion material having an increasing ZT value. As is evident from the above formula (1), a higher Seebeck coefficient, a lower electrical resistivity, and a lower thermal conductivity are demanded of the thermoelectric conversion material.
A PbTe alloy has been used in a conventional thermoelectric conversion material, but Pb (lead) is harmful to humans. On the other hand, a half-Heusler compound having a MgAgAs type crystal phase attracts attention as a thermoelectric conversion material that can be used even at a high temperature and does not contain harmful substances at all or reduces such substances to a minimum. According to Jpn. Pat. Appln. KOKAI Publication No. 2007-173799 (Patent Literature 1), a certain improvement in the ZT value in the half-Heusler compound is achieved by the inclusion of a predetermined composition, but a still higher ZT value is demanded.
In Jpn. Pat. Appln. KOKAI Publication No. 2010-129636 (Patent Literature 2), a half-Heusler alloy whose ZT value is improved by including crystals of different Ti molar concentrations.